Tunneling Measurements of the cuprate superconductors

After a very brief description of what has been learned from tunneling measurements in conventional superconductors, we provide an overview of general concepts relevant to the cuprates. These include the types of junction structures used, effects due to variable junction transparency from the point contact to the tunneling regimes, proximity effects, Andreev scattering, unconventional pairing symmetry, and possible broken time reversal symmetry. We describe the various methods used for obtaining tunneling junctions in the high-temperature cuprate superconductors. We describe how the unconventional pairing symmetry of the cuprate superconductors leads to π-rings and 0–π-junctions, and how these effects have been used to determine that the gap in the cuprates has predominantly dx2−y2 pairing symmetry. We then turn to tunneling spectroscopy. The superconducting gap, the pseudogap, and zero bias conductance peaks are closely interrelated. The superconducting gap and zero bias conductance peaks can be understood in terms of transport between electrodes with dx2−y2 pairing symmetry through low and high transmissivity barriers. It is controversial whether the pseudogap represents an order competing with superconductivity or preformed Cooper pairs. Similarly, there are many indications of broken time reversal symmetry in tunneling spectroscopy measurements, but not in measurements of π-ring and 0–π-junctions. Conductivity modulations in atomically resolved scanning tunneling spectroscopy certainly can arise from quasiparticle interference effects, but there is also evidence for nondispersive conductivity modulations, expected from stripe models. We describe tunneling evidence for strong coupling effects involving phonon and magnon interactions with the quasiparticles in the superconducting state.